Creating 2 Random Matrices in C using malloc and pointers

Question

I have an assignment to create two randomly generated matrices of size n (where n = 4, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192) and multiply these matrices.

I am creating the matrices using malloc. I'm using just long** intCreateMatrix(n) function to create both the matrices A and B. This function returns a pointer to the matrix. Another function long** intInitializeMatirx(a,n) is used to initialize the matrix with random values.

When I call these functions for two different matrices it prints the same contents of matrix for both the matrices.The code is given below:

void intAlgo(int n,int algo){

    long **a;
    long **b;
    long **c;

    a = intCreateMatrix(n);
    printf("Matrix A:\n");
    a = intInitializeMatrix(a,n);

    b = intCreateMatrix(n);
    printf("Matrix B:\n");
    b = intInitializeMatrix(b,n);

}

long** intCreateMatrix(int n){

    long **arr;
    int i,j;
    arr = (long **)malloc(n * sizeof(long *));
    for (i=0; i<n; i++)
        arr[i] = (long *)malloc(n * sizeof(long));
    return arr;

}

long** intInitializeMatrix(long **arr,int n){

    int i,j;
    srand(time(0));
    for(i=0;i<n;i++){
        for(j=0;j<n;j++)
            arr[i][j] = rand()%10;
    }
    for(int i=0;i<n;i++){
        for(int j=0;j<n;j++)
            printf("%ld ",arr[i][j]);
        printf("\n");
    }
    printf("\n");
    return arr;
    free (arr);

}

intAlgo is the function that calls the createMatrix and Initialises matrix.

CreateMAtrix takes n as input and returns a pointer to the matrix created using malloc.

InitialiseMatrix initializes the matrix using rand function and prints the matrix and returns the matrix.

When I execute it a and b should have different values but they have the same values example I'm getting these values for Matrix A and `B? :

Matrix A:

Matrix B:

These two matrices should be different I'm not sure where I am going wrong. Could you please guide me?

Call `srand()` **once** not for every time you call it. – user2736738 Feb 17 '18 at 05:06 — user2736738, Feb 17 '18 at 05:06
Thank you so much :) it worked :) – Aisha Sid Feb 18 '18 at 07:34 — Aisha Sid, Feb 18 '18 at 07:34

user2736738 · Accepted Answer · 2018-02-17T05:16:18.727

srand(int) is used for seeding the pseudo-random number generator. Each time rand() is seeded with the same seed, it must produce the same sequence of values. It should be seeded once before calling rand(). It should not be repeatedly seeded, or reseeded. Call srand(time(0)); once before you start calling initializeMatrix in main().

Also after return statement writing code is meaningless - control never reaches those lines. And why do you want to free it there? Free it after you are done working with it. You still needed them for printing the values stored in those dynamically allocated memories.

Casting the malloc is unnecessary but checking the return value of malloc is not. You should be checking it.

free() is code error i forgot to change it while i was trying it without return type.... btw thank you so much i tried the srand just once and got it :) — Aisha Sid, Feb 18 '18 at 07:35

Creating 2 Random Matrices in C using malloc and pointers

1 Answers1